Tools

Scope

Dual-trace oscilloscope - see your audio and CV in the time domain, sample by sample.

What is a Scope?

An oscilloscope is a measurement instrument that draws a graph of voltage versus time. The vertical axis is signal level, the horizontal axis is time, and the trace shows you the exact shape of the wave - sample by sample, microsecond by microsecond. It does not change anything; it just lets you see what is there. For a synthesist, the scope is what binoculars are for a birder: the tool that turns invisible behavior into something you can read.

The deepest use of a scope is time-domain analysis. A sine wave is a smooth ripple. A sawtooth is a ramp that resets. A square wave is a flat top followed by a flat bottom. A pulse is a square with asymmetric width. An envelope is a slow ramp up, a decay, a sustained level, and a release - drawn out across hundreds of milliseconds. You can verify all of these by ear, but you can verify them *exactly* by eye, in seconds, with a scope.

Most scopes have at least two channels and a trigger system. The trigger is what makes the display stable: instead of redrawing the screen at random moments and producing a smear, the scope waits for the input signal to cross a threshold voltage going upward, then starts the trace from that exact point. A periodic signal (a steady oscillator, a repeating envelope) draws cleanly because every sweep starts at the same phase. Without a trigger, the same waveform turns into a moving blur.

Oscilloscopes have been around since the 1930s - the Tektronix 545A is the lineage that hardware scope-style modules in Eurorack (Mordax DATA, Befaco displays) acknowledge. The DSP is now digital and the screen is now an LCD or a browser canvas, but the gestures are unchanged: time per division, volts per division, trigger level, trigger mode.

Our Scope

Webrack's Scope is 12 HP with two input channels (CH1 green, CH2 cyan), an external trigger input, and pass-through outputs on both channels. The display draws 256 samples per sweep at whatever decimation the Time/Div knob requires, and shows live min / max / peak-to-peak voltage stats below the trace.

Per-channel Gain (0.1x to 10x) and Offset (-5V to +5V) controls scale the trace on screen *only* - the pass-through is always the unmodified signal. That separation matters: it lets you make tiny signals visible (boost the gain to see a millivolt-level audio detail) without changing what your downstream effects receive.

Trigger sources are smart: if the TRIG input has a cable patched, we use it; otherwise we trigger off CH1 automatically. Auto mode periodically forces a sweep so DC and very slow CV are visible; Normal mode waits for real edges and stays blank otherwise. Combined with the on-screen voltage readouts, the Scope is the first place to look when something in a patch is not behaving the way you expect.

In a patch

The most common use is a sanity check. Patch a VCO straight into CH1 and confirm you see a clean sawtooth, not a noisy mess. Patch a slow ADSR into CH1 with the time base at 100ms/div and you can see attack, decay, sustain, and release as distinct phases. Patch an LFO and watch the rate change as you twist the frequency knob.

Two-channel comparison is where the scope earns its keep. Put the source signal on CH1 and the post-filter signal on CH2 - you can literally see the filter changing the waveform in real time. Or put a clock on CH1 and a gate on CH2 to confirm timing relationships. The vertical offset controls let you stack the two traces or align them on top of each other for comparison.

Use the external trigger (TRIG input) for complex signals where neither channel makes a stable reference. A polyphonic mix on CH1 has no clean periodic crossing - patch the underlying clock into TRIG instead, and the display locks to musical time. The same trick works for sync'd FM patches where the carrier frequency varies wildly.

Inputs

CH1 (audio) — Channel 1 input (green trace). Audio (+/-5V) or CV - the scope handles either. Also the default trigger source when nothing is patched into TRIG.
CH2 (audio) — Channel 2 input (cyan trace). Patch a second signal here to compare it against CH1 - common pairings are pre/post a filter, an LFO and the signal it modulates, or two oscillator waveforms.
TRIG (cv) — External trigger input. When patched, this signal replaces CH1 as the trigger source. Useful when CH1 is noisy or DC-offset and the display flickers - patch a Clock pulse or gate here for a rock-stable trace.

Outputs

CH1 Thru (audio) — Pass-through copy of CH1 input, unaffected by Gain1 or Offset1 (those are display-only). Use to insert the Scope mid-chain without altering the signal that reaches the next module.
CH2 Thru (audio) — Pass-through copy of CH2 input. Same contract as CH1 Thru - the scope is non-invasive.

Controls

Time — Time per screen width (1ms to 100ms). 1-5ms for fast audio (look at oscillator waveforms cleanly); 5-10ms for typical audio inspection; 30-100ms for envelopes and LFOs. The full screen width represents this many seconds total.
Trig Lvl — Trigger threshold voltage (-5V to +5V). The display starts a new frame when the trigger signal crosses this level going upward. Sweet spots: 0V for AC-coupled audio (zero-cross); 1-2V to trigger on the rising edge of gates; small positive values stabilize asymmetric LFOs.
Mode — AUTO retriggers periodically even without a crossing - the screen always shows something, useful while patching. NORM only captures on a real crossing, so a quiet signal leaves the screen blank but the trace is stable when the signal returns. AUTO is the right default; flip to NORM when AUTO is jittering.
Gain 1 — CH1 vertical zoom for the display only (0.1x to 10x). Use 5-10x to see millivolt-level signals, 0.1-0.3x to fit big +/-10V swings on screen. Does NOT affect the CH1 Thru output or the voltage readout (which always shows raw input voltage).
Gain 2 — CH2 vertical zoom. Independent from Gain1 - useful when comparing signals of different magnitude (e.g. CH1 audio at 5V, CH2 LFO at 1V).
Ofst 1 — CH1 vertical offset (-5V to +5V). Shifts the CH1 trace up or down on screen. Set to +2V on CH1 and -2V on CH2 to separate two overlapping traces visually.
Ofst 2 — CH2 vertical offset. Pair with Ofst1 to put each channel in its own half of the screen.

Inspired by

A small descendant of the lab oscilloscopes that synthesists have been pointing at their patch cables since the modular era began. The DSP is digital, but the gestures - trigger level, time per division, vertical gain and offset - are exactly the same.

Mordax DATA
Befaco Spring Reverb scope display
Tektronix 545A (the ancestor of every scope)

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